Semiconductor LEDs such as light-emitting diodes and laser diodes are fast replacing conventional light sources in virtually the entire range of light and illumination applications. As a consequence, they are being manufactured in ever increasing numbers for a very wide range of emitted wavelengths.
Example semiconductor LEDs for general lighting are light-emitting diodes (also called “LEDs”) and diode lasers. A phosphor coating can be used to create a “white” light spectrum. The phosphor reacts with a specific emission wavelength of the device (e.g., a blue wavelength) and Stokes shifts a portion of the emission light from shorter to longer wavelengths to give the output light its white spectrum. A white spectrum can be characterized by an equivalent color temperature associated with the emitted light spectrum of the corresponding black-body radiation. A white-light spectrum that is “warm” is characterized by a color temperature of approximately 2800° K, whereas a “cold” white-light spectrum has a color temperature of approximately 5000° K. In a large number of applications, a warm while-light spectrum is preferred.
To obtain the proper color temperature, the emission wavelength λE of the semiconductor light-emitting device needs to be matched to the absorption and emission spectrum Δλ of the phosphor. Typically, the actual emission wavelength λE needs to be within +/−2 nm of the desired (select) emitted wavelength λED to properly match with the phosphor absorption and emission characteristics. Properly matched, the LED lighting fixture provides a “white light” with a color temperature around 2800° K. Devices that fall outside of the particular wavelength specification have considerably less value because they produce light that is “off-color” and hence less desirable by the consumer. An LED manufacturer will often sell these “off-color” LEDs into a less-color-critical application, such as a flashlight, or exterior parking-garage facility. However, the value of these LEDs is much less than those sold to the general household illumination market, where the color temperature is critical. For this reason, the LED manufacturer strives to manufacture more LEDs per wafer that are within the more-valuable spectral range.
For optimum yield and hence optimum value and profit, it is desirable to fabricate the semiconductor light-emitting devices so that they have an accurate emission wavelength to within a specified tolerance. It is also desirable to be able to know beforehand the emission wavelength of the light-emitting chip (die) that goes into the final semiconductor light-emitting device during the manufacturing process.